Contribution of the pentose cycle to lipogenesis in bovine adipose tissue
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The metabolism of 1, 6, and U-14C-labeled glucoses and the incorporation of label from[3-3H]glucose into fatty acids were utilized to calculate the contribution of the pentose cycle to de novo fatty acid synthesis in bovine subcutaneous adipose tissue slices. Due to the low rates of glucose utilization in bovine adipose tissue (less than 1% of those reported for rat adipose tissue), estimates of glucose flux through the pentose cycle that were based on the relative specific activities of triose phosphates were considered unreliable. Estimates based primarily on CO2 production indicated that 12% of the total utilized glucose was metabolized via the pentose cycle in the absence of other exogenous substrates. Acetate and lactate (± acetate) increased this value to 22 and 37%, respectively, but had no effect on total glucose utilization. Based on 14CO2 production from [1-14C]glucose by the pentose cycle and the yield of tritium from [3-3H]glucose recovered in fatty acids, 30% (glucose alone), to 72% (glucose + acetate and lactate) of the pentose cycle-derived NADPH was utilized for lipogenesis. Calculations of glucose metabolism via the pentose cycle and the incorporation of label from [3-3H]glucose in fatty acids also were used to estimate the percentage of NADPH required for lipogenesis supplied by the pentose cycle. Both methods indicated that the pentose cycle provided 7% of the NADPH required for lipogenesis from lactate, and 33 to 43% of that required from acetate (± lactate). NADP-Isocitrate dehydrogenase and NADP-malate dehydrogenase provided the remainder, with NADP-isocitrate dehydrogenase providing 36 to 60% of the reducing equivalents required for lipogenesis, depending on the specific substrate availability. The data indicate that there was little correlation between the rate of incorporation of acetyl units into fatty acids and glucose carbon flux through the pentose cycle in bovine adipose tissue in vitro. © 1983.
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